1. Field of the Invention
The present invention relates to a process for the production of ethylene-α-olefin copolymers having enhanced short-chain branching characteristics. A mixed modifier comprised of a conjugated diene and an alkoxysilane is employed for the copolymerization.
2. Description of the Prior Art
Various organosilicon compounds have been employed with Ziegler-Natta catalysts as selectivity control agents (SCA's), primarily for the polymerization of propylene and ethylene to modify isotacticity. These processes are discussed in detail in PCT International Publication No. WO 2005/005489 A1 which discloses the use of specific mixtures of SCAs, preferably comprised of three silane compounds, for the polymerization of propylene or mixtures of propylene and ethylene.
U.S. Pat. No. 6,642,326 discloses the use of hydrosilane and polysiloxyhydrosilane modifiers to enhance the activity of boraaryl single-site catalysts.
The addition of tetraalkylorthosilicates to gas phase fluidized bed olefin polymerization reactors is disclosed in U.S. Pat. No. 5,731,392 to control static and thus reduce buildup of polymeric material on the reactor walls.
Copending application Ser. No. 11/180,853, filed Jul. 13, 2005, discloses a 2-step polymerization process for the preparation of copolymers having narrowed short-chain branching distribution using a monotrialkoxysilane compound.
Y. V. Kissin in an article entitled “Kinetics of Olefin Copolymerization with Heterogeneous Ziegler-Natta Catalysts,” Macromol. Symp. 89, 113-123 (1995), has studied the effects of independently using arylalkoxysilanes and conjugated dienes with Ziegler-Natta catalysts on the kinetics of ethylene/hexene-1 copolymerizations. The author concludes the arylalkoxysilanes and conjugated dienes poison different active centers in the catalyst. There is no indication by Kissin to the use of silanes and dienes of any type in combination or that any advantage may be realized thereby.
It would be highly advantageous if a method were available to modify the short-chain branching characteristics of ethylene-α-olefin copolymers. It would be even more desirable if such method could be accomplished using known modifiers. These and other advantages are possible with the process of the invention.